I've got a Heavy 10L restoration project sitting here on hold for other reasons, but I tripped over this source of custom belts for SB lathes on a link from another forum and totally different subject. These may be the folks who sell on ebay, but I don't know for sure. They also mention Atlas lathes. I've not seen an Atlas that uses flat belts, but not by any means an Atlas expert, either. HTH!
Bill in OKC
Bill in OKC too